When did Kermit become a frog?

In the early days of the Muppets, the character Kermit was not a frog. He was introduced in 1955 on the show Sam and Friends with roundish feet instead of flippers and no collar. As Jim Henson described: "all the characters in those days were abstract"; Kermit was simply a lizard-like creature, and was not a specific species. Kermit truly became a frog in the late 1960s. However, the exact moment of the transformation has been unclear. Whereas most official sources list the 1971 TV special The Frog Prince, research shows it happened earlier.

Jim Henson: The Works, mentions that Kermit first gained froghood for The Frog Prince. Jim Henson, himself even said so in a 1982 interview:[1]

Jim Henson: ...back in those days you may have read somewhere, but I didn't call him a frog.

Judy Harris: Right, he was just Kermit the thing.

Jim Henson: Yeah, all the characters in those days were abstract because that was part of the principle that I was working under, that you wanted abstract things.

Judy Harris: He didn't turn into a frog until you did The Frog Prince for a TV special.

Jim Henson: Yeah, that's right.

Judy Harris: That was the first time he got flippers and his little pointed collar?

Jim Henson: When [Kermit] was first made he was, all my characters were abstract, he wasn't really a frog. He was a lizard-like character. But he became a frog when we did a show called The Frog Prince.

“When I started doing this little local television show, Kermit was more a lizard-like character. We frogafied him over a couple of television specials we did years ago, before Sesame Street. So he just slowly became a frog. I don't think there was a conscious move to do that...

He's very primitive as a puppet goes because he's really like a glorified sock puppet. But he changed a little bit - a few years after I first made him because when I first made him, he was - all my characters in those days were abstract. And he was sort of a lizard-like character. And then after a few years, we changed and made more of a frog-like body for him and gave him flippers for feet. And that was for a special we did called The Frog Prince that he became more of a frog.”